Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing production of regulated emissions, such as CO2. For example, alcohol and alcohol-based fuel blends have been recognized as attractive alternative fuels, in particular for automotive applications. Various engine systems may be used with alcohol fuels, utilizing various engine technologies such as turbo-chargers, super-chargers, etc. Further, various approaches may be used to control such alcohol-fuelled engines, including adjustment of boost or spark timing in dependence upon an alcohol content of the engine fuel, and various other engine operating conditions.
One example approach to control alcohol-fuelled engines is described by Brehob in U.S. Pat. No. 7,287,509. Herein, the injection timing of a directly injected alcohol fuel is adjusted to take advantage of the increased charge cooling effects of the alcohol fuel's higher heat of vaporization and increased octane. Specifically, the injection timing of one or more direct injections is advanced with increased alcohol in the fuel to take advantage of the higher latent enthalpy of vaporization of alcohol and to allow more time for vaporization. By advancing the injection timing, the intake system is cooled to enable the charge density that can be rammed into the combustion chamber to be increased. Overall, the charge cooling effect of the alcohol fuel is used to improve the peak torque output of the engine.
However, the inventors herein have recognized potential issues with such an approach. In one example, during an engine cold-start, when the temperature conditions of the engine are already not hot enough for an efficient combustion, advancing the injection timing responsive to an increase in fuel alcohol content may further cool the system and significantly reduce the efficiency of fuel evaporation and the formation of a homogeneous air-fuel mixture. The larger amount of time required to evaporate the fuel may degrade engine startability. Additionally, the charge cooling effect of the alcohol fuel on the intake system may further lower the air-charge temperature at cold-start conditions thereby further degrading combustion stability and increasing potential for engine misfire. As such, this may lead to reduced fuel economy and degraded cold-start exhaust emissions.
Thus in one example, some of the above issues may be addressed by a method of operating an engine, the engine including an injector configured to directly inject fuel into an engine cylinder. One example method comprises, during an engine cold start, performing compression direct fuel injection, and retarding a timing of the compression injection as a fuel alcohol content of the fuel increases.
In one example, the engine may be a flex-fuel engine of a vehicle configured with direct fuel injection. During an engine cold-start, when operating the engine with an alcohol-blended fuel, such as during a first number of combustion events from the start of engine rotation, a compression direct fuel injection may be performed. Further, the timing of the compression injection may be retarded as a fuel alcohol content of the injected fuel increases. In one specific example, during a first cold start of the engine, compression injection may be used for the first fuelled cylinder from the engine start, with the compression injection timing being a first timing where the fuel has a first alcohol content. During a second cold start of the engine, for example on a second day when the fuel blend in the fuel tank is different, compression injection may be used for the first fuelled cylinder from the engine start, with the compression injection timing being a second timing (more retarded from the first timing) where the fuel has a second alcohol content (higher than the first alcohol content, e.g., the second fuel has a greater alcohol amount by weight than the first fuel).
In one example, when operating with a fuel-blend with a lower percentage of alcohol (such as E10, which has 10% ethanol and 90% gasoline), the compression injection timing may be less retarded (that is, the injection timing may be less close to TDC of the compression stroke than to BDC). In another example, when operating with a fuel-blend with a higher percentage of alcohol (such as E85, which has 85% ethanol and 15% gasoline), the compression injection timing may be more retarded (that is, the timing may be moved closer to the TDC of the compression stroke). Additionally, to improve the atomization of the alcohol-based fuel, the fuel rail pressure may be raised with increasing alcohol content of the fuel. Further, to reduce potential over-pressure related issues, multiple compression fuel injections may be performed, the number of injections increased with the increasing alcohol content of the fuel.
By performing compression direct fuel injection, a higher air-charge temperature and higher valve temperature of the engine cylinders during the compression stroke may be advantageously used to more effectively evaporate the directly injected alcohol fuel. Since alcohol is a relatively low volatility fuel, by retarding the compression injection timing as the fuel alcohol content increases, the alcohol may be exposed to higher air-charge temperatures, thereby better enabling efficient evaporation and formation of a homogenous air-fuel mixture. In this way, the startability of alcohol-fuelled engines may be improved. Additionally, by evaporating most of the injected fuel, less fuel may be lost during engine operation, and the need for larger or pilot fuel injections at engine cold-start may be reduced. As such, this may provide fuel economy benefits as well as reduced cold-start exhaust emissions. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.